In an optical storage system, data on an optical disk is stored in spiral or concentric tracks. A laser beam is directed through a series of optical elements and focused as a beam spot on a surface of the disk. Typically, a coarse carriage, on which is mounted an objective lens through which the beam passes, is moved along a radial path to enable the beam spot to be moved between the innermost and the outermost track on the disk. The resolution of the coarse carriage and controlling servo loop is generally sufficient to permit the beam spot to be positioned only within a few tracks (such as between about .+-.5 and about .+-.50 tracks) of a desired target track. Consequently, a fine tracking actuator is employed to supplement the coarse carriage by finely controlling the beam spot to position and maintain it on the target track.
Because the range of motion of the fine tracking actuator is very small (less than about .+-.0.06 mm), the motion of the fine tracking actuator must be coordinated with that of the coarse carriage during track seeking and following operations. Moreover, the position of the beam spot due to the motion of the fine tracking actuator must be known relative to the coarse carriage. A relative position sensor associated with the fine tracking actuator generates a relative position error (RPE) signal to provide the drive controller with this necessary information.
As will be appreciated, it is desirable that the RPE signal be substantially linear over the range of interest and have a known signal-to-position slope. Many existing optical drives include physical stops which limit the range of motion of the fine tracking actuator. The RPE signal gain can be calibrated with reference to the stops such that the voltage of the RPE signal is at a known level at each of the two stops.
The fine tracking actuator of more recently developed optical drives, such as those employing galvanometer mirror fine actuators, may not include physical stops and, therefore, the RPE gain cannot be calibrated in the same manner.